#include<iostream>
#include<iomanip>
using namespace std;
double arr[100];
double brr[100];
int main()
{
	int n;
	while (cin >> n && n)
	{
		for (int i = 0; i < n; i++) {
			 cin >> arr[i];
		     cin>>brr[i];
		}
		arr[n] = arr[0];
		brr[n] = arr[0];
		double s = 0;
		for (int i = 0; i < n; i++) {
			s += arr[i] * brr[i + 1] - arr[i + 1] * brr[i];
		}
		cout <<fixed<<setprecision(1)<< s/2.0 << endl;
	}
	return 0;
}